Anhydride cured epoxy resins, alone, and in admixture with catalytic copolymers, have been used in liquid resin casting and potting operations, to provide insulated, electrical bushings, coils, and the like, as described, for example, by Smith, in U.S. Pat. No. 4,349,651. It has been well known for many years that the dielectric strength of cured organic resins, such as cured epoxy resins, and filled compositions in which they are a part, decline in dielectric strength under voltage stress, particularly a-c voltage stress. This is attributed to the effect of partial discharges on the surface or within internal cavities of the cured resin.
When resins are vacuum cast around metal electrodes or conductors, internal cavities are reduced or nearly eliminated, but not always completely. Cavities may occur as a result of differential shrinkage of the resin with respect to the metal conductors, upon thermal cycling, if the resin, or filled resin thermal expansion coefficient is not completely matched to that of the metal. Cavities may also occur due solely to resin shrinkage during polymerization, where unfilled epoxy resin, for example, exhibits about 5% shrinkage, and filled epoxy resin, exhibits from about 1% to 2% shrinkage.
When the cavities are not too small, the partial discharges within them under voltage stress can be detected. Better degassing and vacuum processing of the resin, and use of extreme care in handling the conductor parts and mold, can reduce the chance of cavities and their size. However, even with the best practical processing, it has been the experience that cast prior art resin systems have a finite voltage endurance which limits the voltage stress at which they can be used in service. This limitation can be indicated by exposing the insulated, vacuum cast conductors or electrodes to high voltage stresses for long periods of time.
Gore, in U.S. Pat. No. 3,278,673, recognized shrinkage caused cavity problems in polytetrafluoroethylene insulated, 25 mil. (0.06 cm) diameter wire. Gore, in an attempt to alleviate this problem, soaked 4 mil. (0.01 cm) thick, substantially unsintered, porous tape, consisting of polytetrafluoroethylene resin particles, in a high boiling, liquid dielectric fluid, such as poly(methyl, phenyl)siloxane or polymethylsiloxane, and then wrapped three turns of the tape around the conductor, to provide a total insulation about 12 mil. (0.03 cm) thick. This was followed by sintering the polytetrafluoroethylene resin particles at about 360.degree. C. The sintering caused 25% shrinkage, and collapse of the spongy, cellular structure of the unsintered polytetrafluoroethylene resin, trapping the liquid dielectric fluid uniformly throughout the resin tape multiple layer cross-section. If a cavity near the conductor was enlarged by a-c voltage stress, the nearby dielectric fluid droplets controlled such cavity growth.
The Gore system is designed for thin insulation layers. What is needed, however, is a low shrinkage, liquid casting composition useful for thick resin insulation systems, where most void problems, upon thermal cycling or under voltage stress, are most likely to occur at the conductor-resin interface.